Single-crystal silicon carbide (SiC) is a wide band gap semiconductor having a broad forbidden band width of 2.2 to 3.3 eV. Owing to its outstanding physical and chemical properties, SiC has long been a focus of R&D for its potential as an environmentally rugged semiconductor material. In recent years, single-crystal SiC has attracted increasing attention as a wafer material for short wavelength optical devices in the blue-to-UV spectral region, high-frequency electronic devices, high-voltage electronic devices and the like, and R&D in these areas has become increasingly active. In the semiconductor field, large-area single crystal of high quality is required for realizing industrial-scale production. However, no technology for reliable supply of large-diameter single-crystal SiC of high-quality has yet been established.
Growth of single-crystal SiC of a size suitable for fabrication of semiconductor devices has been possible on a laboratory scale using, for example, the sublimation growth process (Lely process). However, the single crystal obtained by this method is of small area. In addition, its dimensions, shape, polytype, and carrier concentration are not easy to control. On the other hand, cubic single-crystal SiC is being carried out by heteroepitaxial growth, i.e., growth on a substrate of a different type like silicon (Si), using chemical vapor deposition (CVD). Although large-area single crystal can be obtained using CVD, only single-crystal SiC containing many defects (up to 107/cm2) can be grown because of, inter alia, the large (about 20%) lattice-mismatch between SiC and Si. That is, high-quality single-crystal SiC cannot be obtained.
The modified Lely process, which conducts sublimation growth using a single-crystal SiC wafer as a seed, was developed to overcome these problems (Yu. M. Tairov and V. F. Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp. 146˜150). Owing to its use of a seed crystal, the modified Lely process can control the crystal nucleation process and, by controlling the ambient inert gas pressure to around 100 Pa to 15 kPa, can control crystal growth rate with good reproducibility.
The modified Lely process makes it possible to grow single-crystal SiC while controlling its polytype (6H, 4H and 15R and other polytypes), shape, and carrier type and concentration.
Currently, 2-inch (50 mm) to 4-inch (100 mm) single-crystal SiC wafers are being cut from single-crystal SiC grown by the modified Lely process for use in fabricating devices and the like in the power electronics and other sectors. In most cases, however, the crystals are observed to contain micropipes (hollow hole-like defects extending in the longitudinal direction of the crystal) at the rate of up to around 100/cm2. Moreover, it is also known that in the conventional single crystal growth methods, polycrystalline SiC growing around the single crystal comes in contact with the single crystal on the seed crystal. This produces strain in the single crystal that degrades its quality. As pointed out in P. G. Neudeck, et al., IEEE Electron Device Letters, vol. 15 (1994) pp. 63-65, the micropipes cause leakage current and other problems in a fabricated device. Mitigation of such drawbacks is the overriding issue in the application of single-crystal SiC in devices.
In order to inhibit such degradation of crystal quality it is important to optimize the temperature gradient in the crucible. A temperature gradient whereby the peripheral region of the growth ingot is higher than its interior is known to be effective. Moreover, as reported in M. Selder, et al., Journal of Crystal Growth, vol. 226 (2001) pp. 501-510, the temperature gradient of a single-crystal ingot generates thermal stress inside the crystal. This thermal stress poses a major problem if large, because it may induce dislocation defects and cause ingot cracking. Precise control of the temperature gradient inside the crucible is therefor essential for obtaining a good-quality single-crystal SiC ingot by the sublimation growth process.
Up to now, the method most generally used to control the temperature gradient in the crucible in the sublimation growth process has been to regulate the positional relationship between the crucible and the induction coil. However, this method not only changes the temperature gradient at the crystal growth zone but also simultaneously changes the crystal and feedstock temperature gradient and the point of maximum crucible temperature. This makes precise temperature gradient control difficult. For example, when the crucible is positioned near the middle of the induction coil in an attempt to increase the temperature at the growth surface by this method, feed gas ceases to be supplied in the direction of the seed crystal, so that growth is interrupted and the crystal surface carbonizes.
A solution to this problem is taught by Japanese Patent Publication (A) No. 2004-224666, for example. The invention of this publication is directed to fine control of the crucible internal temperature by disposing multiple induction coils equipped with resonant inverters at the crucible seed substrate region, intermediate region and SiC powder material region. However, the invention has not been able to achieve its purpose of finely and independently controlling the feedstock and growth crystal because the induction coils also affect the heating of regions adjacent to the crucible.
S. Nishizawa, et al., Materials Science Forum, Vols. 457-460 (2004) pp. 29-34 and Japanese Patent Publication (A) No. 2005-53739 teach prevention of generation of polycrystal from the vicinity of the growth crystal by using the internal structure of the crucible to control temperature gradient and simultaneously forming a flow of sublimation gas at the ingot periphery. Although the method proposed by these references produces good-quality single crystal it is inferior in productivity because the single-crystal ingot inevitably assumes a shape of large diameter in the growth direction and, therefore, the length of crystal having the desired diameter is slight or extensive processing of the outer shape is necessary to give it the required diameter.
Some degree of control of the temperature gradient of the crucible interior is also possible by the simple method of locally increasing the thickness of the heat insulation material installed around the crucible. However, this method cannot finely control temperature of the crucible interior because it regulates only the radiation of heat from the outer wall of the crucible.